1. Field of the Invention
The present invention relates to field driven ion propulsion systems, and particularly to methods for maintaining operation of such propulsion systems.
2. Description of the Related Art
Although the principle of ion propulsion was established many decades ago, it has only relatively recently been reduced to practical applications. Ion propulsion generally involves employing an ionized gas accelerated electrically across charged grids to develop thrust. The electrically accelerated particles can achieve speeds of approximately 30 km/second. The gas used is typically a noble gas, such as xenon. The principal advantage afforded by ion propulsion systems over conventional chemical propulsion systems is their very high efficiency. For example, with the same amount of fuel mass an ion propulsion system can achieve a final velocity as much as ten times higher than that obtainable with a chemical propulsion system.
Unfortunately, the range of ion propulsion applications is narrowed by the fact that, although they are efficient, ion propulsion systems develop very low thrust when compared with chemical propulsion systems. However, ion propulsion is well suited for space applications where low thrust is often acceptable and fuel efficiency is critical. More and more ion propulsion is becoming a component of new spacecraft designs. Spacecraft, including satellites as well as exploration vehicles, are presently making use of ion propulsion systems.
For example, ion thrusters are currently used for spacecraft control on some communications satellites. Some existing systems operate by ionizing xenon gas and accelerating it across two or three charged molybdenum grids. As the ions pass through these grids, small amounts of molybdenum are sputtered off to deposit on the downstream grids. Over time, these deposits can grow large enough to flake off and cause a short between the grids, shutting down the thruster. When this occurs, the thruster must be turned off so that the grids can be cleared, removing the short. Specialized grid clear circuitry is employed to apply a large voltage through the short, causing it to blow open.
FIG. 1 is a schematic diagram of a contaminated ion propulsion grid within an ion thruster. When the thruster is operating, ionized gas 102 is accelerated across two or more charged grids 104. However, deposits can accumulate on the grids 104 creating to a point where a short 106 is created, shutting down the thruster.
Prior art grid clear circuits employ a dropping resistor 110 coupled to a fixed voltage source 108 (e.g., the spacecraft bus voltage) to clear the grids 104. The voltage source 108 is applied (through the dropping resistor 110) to the shorted grids 104 for a predetermined length of time. However, this approach delivers varying amounts of energy depending on the resistance of the particular grid short 106. Also, using this approach, the grid clear circuit must be designed to accommodate the worst case grid short 106, without damage to the thruster. With this method, only shorts with a resistance in a limited range can be effectively cleared. Consequently, the amount of energy that can be delivered to very low or very high resistance shorts is limited, making these shorts particularly difficult to clear. On orbit experience has shown that the low resistance shorts are the most predominant type.
Ion propulsion on the NASA Deep Space One spacecraft implements a grid clear by switching a discharge power supply output across the grids to be cleared. Timing of the grid clear procedure is manually controlled through spacecraft commands. However in this case, the timing of the grid clear pulse is predetermined based only on an estimate of the short resistance. If the timing is too long the thruster hardware will be damaged, and if the timing is too short, the grid clear will not be effective. However, the only short ever experienced on this mission was cleared through natural thermal cycling, and not by use of the grid clear circuitry. Consequently, the grid clear has never been attempted.
In view of the foregoing, there is a need in the art for methods and devices to safely and efficiently clear shorts in ion propulsion grids. There is also a need for such methods and devices to deliver a consistent amount of energy, independent of the short resistance. Particularly, there is a need for such methods and systems to clear low resistance shorts. The present invention satisfies all these needs.
Embodiments of the present invention employ a power supply to provide a monitored amount of electrical energy to the grids of an ion thruster to clear any potential shorts. The power supply is designed as a current source. In addition, a timer is used to limit the total energy into the thruster grids to prevent damage of the thruster and associated hardware. The timer monitors the output current and/or voltage of the power supply and automatically turns it off to prevent damage.
Thus, embodiments of the invention will enable much larger energies to be delivered to a thruster grid short without damaging the grids. Furthermore, because the grid clear circuitry automatically limits the total energy, the risk of hardware damage from improper spacecraft commands is eliminated. Also, the power supply design can be optimized to clear low resistance grid shorts, which have proven difficult to clear with the prior methods.
Embodiments of the invention can be used in any application of ion propulsion where particulate accumulation requires a grid clear to optimize operation of the propulsion system. Any ion thruster can use this invention to clear grid shorts that normally occur as a result of ion thruster operation.
A typical method embodiment of the present invention includes applying a power supply to the grid to clear contaminants, monitoring the energy applied to the grid by this power supply and suspending application of the power supply to the grid after the monitored energy substantially reaches a predetermined value. A typical device includes a controller for applying a power supply to a grid to clear the grid of contaminants and a timer for monitoring the energy applied to the grid by the applied power supply and suspending application of the power supply to the grid after the monitored energy substantially reaches a predetermined value.